In equipment used at a high voltage such as a transformer, a motor, a switch gear, a power transmitting and transforming equipment, a high voltage element and others, it is necessary to insulate the circuit conductors provided in the equipment or the grounds from each other. To this end, a thermosetting resin having excellent electric insulation has been used as an insulating layer. As a thermosetting resin suitable for such an insulating material of a large-sized high voltage electrical equipment, a thermosetting epoxy resin is generally used because it is inexpensive and has high performance as an electrical, insulating resin. Furthermore, as the insulating resin material, an epoxy resin-based molding material which contains a curing agent and an inorganic filler such as silica has been used.
Particles of a predetermined concentration or more are added to the thermosetting resin so as to improve characteristics of the resin such as fracture toughness, high strength, thermal stability, breakdown resistance, long life, environmental load and the like. On the other hand, a viscosity of the thermosetting resin significantly increases due to a polarity thereof particularly when hydrophobic fine particles of 100 nm or less are present in a raw material liquid of the thermosetting resin. Here, when the viscosity is increased due to the presence of the fine particles in the raw material liquid of thermosetting resin such as an epoxy resin, the resin loses fluidity and it leads to a problem of the occurrence of voids which cause a significant reduction in a mechanical strength. Here, when the voids occur in the resin, the discharge in the voids which is referred to as a partial discharge is induced, resulting in the destruction of the equipment, and thus the voids are not preferable particularly in the high voltage equipment. In addition, in the case where the voids are present in the resin, there is a problem that the vacuum molding cannot be performed if the vacuum molding is to be performed.
For example, Patent Document 1 discloses an invention that relates to a semiconductor sealing epoxy resin composition applicable to a semiconductor device. Specifically, Patent Document 1 describes a thermosetting resin composition containing 0.01 to 5.0 parts weight of a surfactant having Hydrophile-Lipophile Balance (FISH) of 10 or more in addition to the epoxy resin. However, in the hydrophilic surfactant in which a limit of an HLB value is 10 or more, it is not possible to improve the dispersibility of fine filler (hereinafter, referred to as nanofiller) of 100 nm or less in particular in the epoxy resin. In addition, in the case of using the surfactant with a high HLB value, it is not possible to lower the viscosity of the resin, which causes the problem that the resin material does not flow through a pipe in the case of performing the vacuum molding and voids occur in a molded resin.